In general, polyesters such as polyethylene terephthalate are excellent in various physical and chemical characteristics, and hence, they are used in various applications, for instance, fibers, moldings and films such as base films for a magnetic recording tape, floppy disc, photography, condenser, packaging, roentgen film, micro film and the like. In such uses, polyester films are required to be sufficient in slipperiness, abrasion resistance, etc., because these properties afford a serious influence on the workability in film production and processing for various applications, and the quality of the produced films.
Particularly, when a polyester film is used for manufacture of a magnetic recording tape, friction and abrasion are remarkable between the film surface and a coating roller on application of a magnetic coating composition onto the film surface, which result in production of wrinkles and scuff marks on the film surface. Further, the resultant magnetic recording tapes, which are used for audio tapes, video tapes, computer tapes and the like by slitting a film coated with magnetic coating composition, move slidably in close contact with many guide rolls, a reproduction head, etc. Thus, friction and abrasion therefrom are so great that scuff marks and strain on the film are readily produced during winding off and winding up by reel or cassette. In some cases, the film surface is scraped off to afford white powders and these may cause production of drop-out, which is the lack of magnetic recording signals.
Further, when a polyester film is used repeatedly as a magnetic recording tape, the running characteristics thereof are deteriorated by an increase of the friction coefficient between the tape and metallic guide rollers or the like with the number of running times. Then, in an extreme case, the guide rollers roll in the tape, or voice (sound) is caused between the tape and reproduction head or guide rollers.
In order to improve the running characteristics such as slipperiness, abrasion resistance and durability of a polyester film, a number of minute concaves and convexs are formed at the surface of the film so as to decrease the contact area of the film with the guide rolls etc. Formation of said concaves and convexs may be accomplished, for instance, by a procedure wherein insoluble particles are separated out from the catalyst residue as a result of polymerization, a procedure wherein inactive inorganic particles are incorporated into the polymeric material, etc.
Usually, particles having a larger particle size produce higher enhancement of slipperiness. In case of a magnetic recording tape, particularly precision films such as video film, however, particles of larger size in the film afford larger projections or concaves and convexs in the magnetic layer, and spacing loss is produced between the magnetic recording tape and a magnetic recording head during running, whereby the lack of magnetic signals, i.e. drop-out, results. It is thus desired that the size of particles in the film is decreased so as to make the concaves and convexes on the surface of the film as minute as possible. Thus there arises an antipodal requirement with a recording tape.
In particular, a magnetic recording tape, which needs a high S/N ratio, such as video tapes of the metal coating type and the vacuum evaporation type, has a remarkably thin magnetic layer, and hence, a shape on the surface of the base film employed directly appears as that of a magnetic layer. Thus, when the flatness of the base film is low, magnetic transducer characteristics of video tapes produced by coating or vacuum evaporation of magnetic composition therefrom are remarkably inferior. Therefore, in such applications, the flatness is particularly required without failure of slipperiness and abrasion resistance.